It is known that for many uses, particularly for the construction of gas distribution systems, pipes made of plastic, especially polyethylene, are employed more and more frequently.
These pipes cannot be joined together satisfactorily by adhesive bonding, and it is therefore necessary to weld them, more specifically weld them thermally.
A technique practiced more and more involves joining the pipes together by means of sleeves, i.e. fittings or pipe joints, which have an electric winding on their inner face.
The welding operation is carried out by connecting the winding to an electrical supply source which, by the Joule effect, will heat the winding and cause the fusion of the plastic of the sleeve and of the corresponding pipe, thus carrying out the welding.
Other parts, such as branch taps, plugs, etc., are likewise joined to the system on the same principle.
It is also necessary to know that, at the present time, there are several manufacturers of parts, made especially of polyethylene, which are joined together in this way by electric welding.
Each manufacturer has drawn up exact rules for welding his equipment under good conditions.
However, today these rules are often based on a welding technique in which, on the one hand, the only temperature readings taken to assist the smooth progress of the welding operation are those carried out directly on the parts to be welded and, on the other hand, the electrical source supplies to the terminals of the sleeve an energy, that is to say a voltage or a current, which is substantially constant in terms of effective value for the entire duration of the recommended welding time.
As will be appreciated, keeping the winding under a constant voltage (or current) for the entire duration of the heating causes a continuous rise in its temperature, until the welding time has elapsed.
Each manufacturer is therefore obliged to determine accurately the ideal duration making it possible to weld his equipment. Moreover, this welding time must be adhered to strictly in practice on site.
In fact, it was shown that this welding method had various disadvantages which could affect the quality of the weld.
When the adopted welding time is too short, the rise in temperature is insufficient and the weld is of mediocre quality.
When this same time is too long, there is the fear of damage to the plastic which forms the parts to be welded and which is brought to a high temperature, and short-circuits between the turns of the winding can occur.
Furthermore, it was noted that poor preparation of the pipe (irregular or too superficial scraping, etc.) and the presence of play between the pipes to be welded and the sleeve had a considerable influence on the quality of the weld in this case.
Consequently, the current welding technique just described makes it necessary, most of the time, to carry out a correction of the welding time as a function of the initial temperature of the parts to be joined together or to provide a narrow production tolerance on the value of the electrical resistance of the winding of the sleeve.
The object of the invention is to avoid the difficulties which have just been mentioned by enabling the user to carry out the welding under the best possible conditions, in particular without running the risk of damaging the parts to be welded as a result of a rise of the welding temperature beyond the temperature at which the plastic forming these parts is damaged.